Four hundred sounds like a good buy, depending on condition, etc. Lots of work to remove, but you will find out some interesting new stuff about the MB cars when you get into it that deep.
The SD interior parts are a bit rare on the used market. Lots of call for some of the small stuff, like the door parts, handles, and so on. But to sell the entire interior at one time would be a good move on your part. Get it over with and put the $ into your next project.